When a poet friend invited Redmond O'Hanlon on a trip to Borneo, it proved the beginning of a new life as an acclaimed travel writer. By Andrew Brown.

Caddis larvae build their houses at the bottom of slow-moving rivers. Slow-moving, they work all their lives to agglomerate twigs, little flakes of stone and any other detritus of the river bed. They chew on these and bind them together with a gluey secretion to make a sturdy patchwork house. You cannot pull a caddis from its house; if a trout wants to eat it, he must swallow the whole thing.

In 1983, Redmond O'Hanlon must have seemed like a caddis. He had lived for 15 years in a house outside Oxford with his wife Belinda. They had never spent a night apart since their marriage, and the patchwork fortifications he had assembled around them looked quite impregnable - as they still do today.

Writers' houses are normally full of books, and his is too, but books are only the start of the clutter. On a shelf is a glass jar containing part of the charred foot of his best friend at school, who committed suicide at 24 by burning himself to death; there is also a furry fetish object containing fingerbones - possibly human - from the Congo.

In 1981, at a party in the house, the poet James Fenton suggested O'Hanlon come on a jungle-walking holiday in Borneo. O'Hanlon was then natural history editor on The Times Literary Supplement and in the grip of a wrecking depression after finishing his doctoral thesis on Joseph Conrad and Charles Darwin. He was invited because Fenton wanted someone along who would be able to identify all the flora and fauna of the jungle.

O'Hanlon turned up at Heathrow in camouflage fatigues with £2000-worth ($4700) of overweight baggage, and turned the holiday into a horribly dangerous odyssey into the jungle, in search of a rhinoceros with a crosspiece on its penis - an animal that might very well be extinct. When the journey was done, he took flight into a book so funny and frightening it looks quite artless. Just so the caddis, which has worked all its life to be so closely armoured, emerges one day from its wonderful shelter. It has been transformed and leaps into the air and flies off like a cultured butterfly.

Into the Heart of Borneo was published in 1985 and made O'Hanlon's name at once. It was, said travel writer Eric Newby, one of the three best books of its kind published since the war. O'Hanlon dragged himself, his canoe and Fenton, into some very dangerous and unexplored places. They paddled and walked to the edge of the map, to the place where their SAS-supplied charts warned "No cover. Area; Unsurveyed; Relief data incomplete; Map sources unreliable" - and then they found nothing but birds, jungle and discomfort.

Most travel writing, and most exploration, nowadays, has a phoney quality. People set off to cross Antarctica by unicycle, or get from Tucson to Tucumcari on a pogo stick. They wouldn't do it if no one were watching. But O'Hanlon seems driven by the same combination of hope and misery that drives a child at boarding school out into the surrounding countryside. He travels as a lonely atheist, with a bodyguard of ghosts.

Physical danger is turned into comedy, but the moral dangers are played straight. In a longhouse in inner Borneo, with pigs rooting under the floor, he is taken to see a woman with gangrene. "As my eyes adjusted, I looked where everyone was looking: at her foot. My stomach turned again. The top surface was an open pool of fluid with a clearly defined, raised shoreline of indented flesh. She moved slightly as she fanned herself and, as she did so, yellow and black and red islets of infection slithered gently to new positions on the watery surface of the wound. It was a terrible moment . . . she needed massive doses of penicillin, far more than we possessed. I gave her two tubes of Savlon, two packets of multivitamins and a roll of bandages. In return, the old man gave me three sweet potatoes, which I took. It was the nastiest transaction of my life."

What lifts the story beyond "travel pornography" is not just the quality of the writing. It is the offer of the sweet potatoes, and O'Hanlon's knowing that he can't just brush them away. He's not helicoptering in and out. He travelled till he was level with the dying woman and her family, exchanging ludicrously inadequate gifts in the face of death.

It is a very 19th-century story and the Victorian streak in his character is pronounced. His library is full of works of Victorian anthropology and biology; he will sometimes, warmly use "English" as a term of the highest affectionate praise. But he can be quite shocking in his hatred and scorn of the kind of conventional Englishman whose mannerisms he seems to have inherited.

He was brought up in the properly heartless way for a gentleman. His father was a country parson who had wanted to be a flying officer, but his eyesight was too poor. "He was chaplain at Biggin Hill in the Battle of Britain, and nothing ever lived up to that." His mother had been an actress. "She saw him walking in his uniform through Guildford Cathedral and she fell in love. Later she would claim she had given up a huge career for him: 'I could have been' - wait for it - 'someone like Sybil Thorndike'!"

O'Hanlon has inherited a magnificently stagey presence, to be summoned when he wants it, and the disdain with which he regards his parents now is quite superb. His mother would beat him with the flat of a hairbrush to show that boys mustn't cry, a lesson not entirely learned before he was sent away to prep school at seven. As a leaving present, his father gave him a two-volume bird book and the binoculars that had belonged to a great late-Victorian birdwatcher.

"That's how biology began in this country: in natural theology. You were praising God's work as much by cataloguing earwigs as by visiting the parishioners."

Prep school seems to have been no worse than usual; the headmaster received parents in a room furnished with a display of his 16 canes, including one special implement "for hitting seven-year-olds on the buttocks so it did not rupture them". It is difficult to think of another education that would so well prepare a writer, when he finds a monkey's skull in his soup, to suck the eyeball out. This happened on his expedition to Venezuela. Only O'Hanlon would suck out the other eyeball for good measure. He kept the empty skull as a souvenir.

From prep school he went to Marlborough College on a scholarship for the sons of the clergy. It was a curious school; the hedgerows and forests around it were home to adolescent boys looking in nature for a respite from the savagery of civilisation. For the young O'Hanlon it was the place where he was released from Christianity. Until he discovered Darwin, O'Hanlon had still supposed that God was necessary to account for the glory of the world and especially of its birds.

He came to see that calling the Holy Spirit a dove got matters quite the wrong way round; the Holy Spirit does not exist and birds in fact are everything that God is supposed to be in mythology: overwhelming beauty, ultimate reality and the promise of escape from earthbound misery.

He had wanted to read biology at Oxford but could not do the maths. He switched to English, which he regarded as easy, and showed off his prowess in his first year by writing a novel, which got him sent down for a year because it was considered pornographic. By then he was in love with Belinda Harty, a farmer's daughter reading English at St Anne's College. The couple have two children, Puffin, 18, and Galen, 15, both still at school.

He spent a lot of time with the biologists but still did enough work on his English to get a distinguished first, and a job as a junior don, teaching, so he thought, 19th-century literature. It came as a shock when the exams revealed that the syllabus had been extended into the 20th century; and as a greater shock to his students. He was sacked.

His talents and education meant that he was perfectly suited to his part-time job at the TLS. It hardly brought in any money, though. The doctorate on Conrad and Darwin took him seven years and left him in a state of depression for a year. It is definitely the work of a laborious caddis larva and not the accomplished creature he was to become. The transformation, Fenton says, took place within a couple of days of their departure from Heathrow. "He hatched into a Red Admiral, no, a Monarch, and flew magnificently."

This must have been rather disconcerting. There is a magnificently funny set-piece in Borneo in which Fenton is swept away down a rapid and nearly drowned: "James's bald head, white and fragile as an owl's egg, was sweeping round in the whirlpool below, spinning, bobbing up and down in the foaming water, each orbit of the current carrying him within inches of the black rocks at its edge."

In fact, Fenton is rescued only by the strength and heroism of their native guides. But there is far more space devoted to the delights of the birds along the sides of the rivers than to the perils within them.

This does not seem to have occurred to O'Hanlon as odd until he decided, five years after the Borneo trip, to explore another jungle, this time on the upper reaches of the Orinoco in Venezuela. He asked Fenton to accompany him again. " 'James,' I said, 'you are looking ill. You are working far too hard writing all these reviews. You need a break. Why don't you come to the Amazon with me?

'Are you listening seriously?'

'Yes.'

'Are you listening comfortably?'

'Yes.'

'Then I want you to know,' said James, shutting his eyes, and pressing his palms over his face and the top of his bald head, 'that I would not come with you to High Wycombe.' "

Fenton has often been asked whether this story is true. He thinks he may very well have said it. The point, however, is that it had nothing to do with O'Hanlon's prowess as a traveller. It was about the impossibility of two writers going on the same trip when one was destined to be immortalised as the other's straight man or sidekick.

Instead, O'Hanlon took another Oxford friend, one even less prepared for a journey into the wilderness, Simon Stockton. By the time he was recruited for the Amazon trip, he was managing a casino in Knightsbridge.

"I'd read the Borneo book, but I thought, 'James? He can't handle it and I can.' I went without the faintest idea of what I was getting into: when you travel with Redmond you end up shit creek before you realise it. On those trips he's nothing like the bumbling buffoon or affable clown he is at home. You lose comradeship, strangely enough, even though there are only two of you, because he's serious. He can also sleep for 12 hours every night, which is very handy."

Half way through the expedition, after weeks spent chopping their way through a swamp, where the channel was continually blocked by fallen trees, Stockton finally cracked with a magnificent speech that ended: "It's nothing but rain and mosquitoes and the same bloody awful trees and endless rivers and disgusting food and being wet all the time. There's no comradeship. There's no wine and no women and no song and nowhere sensible to shit."

"No way I went quite as mad as I did in the book," he says today. "No way things were quite as bad."

In Trouble Again (1989) was largely funny because of the presence of Stockton, muttering erotic imprecations to his next (and present) wife into a tape recorder in the prow of a canoe being paddled towards a tribe of homicidal Indians. The Yanomami, whom O'Hanlon sought, were dangerous and frightening. But they knew it and they had a code. If they did not kill you, it was a compliment.

The next expedition, to the Congo in 1989, was very much darker. The random dangers and the cruelties had a banal inescapability. In Venezuela, the killers were found deep in the rainforest, and the government was largely peaceful. In the Congo, the moments of greatest danger came when he was closest to the authorities, and the threat of arbitrary violence increased the further he was from the Pygmies in the forest.

Most of the critics still saw Congo Journey as an attempt at humour, but the jokes are brutal. The encounters with corrupt and crazy bureaucrats are horribly frightening and within the first few weeks O'Hanlon nearly died from falciparum malaria.

Beautifully dead - "The final steps down to Les Bougainvilles set off a slap-slap of pain inside my skull, like water in a bucket. The back of my neck locked rigid. My intestines seemed to uncoil and move beneath my stomach like a sidewinder." Dead with bad jokes - "It was worse than the worst hangover I had ever had (but then so was the red wine) . . . Contorted on the lavatory-seat, I decided that this was the worst diarrhoea I had ever had (but then so was the sausage)." But he was very nearly dead, nonetheless.

As soon as the boat goes up river, they watch a young man drowning when he falls from his canoe into the main current of the Congo. No one attempts to rescue him, and O'Hanlon is rebuked by his guide, Dr Marcellin Agnagna, for his distress at the sight.

Agnagna finds a girl in every place they stop because, as he explains, his wife is pregnant, so she can't have sex. He is cultured, a scientist of talent with little chance of work and no chance of serious teaching. The only part of his conservation program the villagers understand or put into practice is to kill gangs of foreign poachers.

O'Hanlon's other companion here was Lary Shaffer, whose upbeat Yankee pragmatism is gradually worn down. "Lary can't read the book," says Belinda O'Hanlon, with pride in her husband's accomplishment. "He wakes up screaming in the middle of the night."

Shaffer says this isn't quite true. "Actually I've got better over time. I still can't read it, but I think about it less. The book is fairly accurate about the hopelessness."

The end of the book has a horrible poignancy. Instead of the moment of revelation, or triumph at the end of the quest, which is meant to round off a travel book, Congo Journey (1996) finishes with O'Hanlon paying off two of his guides. One, who had wanted to be a scientist himself, gives O'Hanlon a bronze circlet. In exchange, "horribly afraid I'd burst into tears", O'Hanlon gives him some money. " 'I'll keep it,' he said, looping the gym bag over his right shoulder, standing up to go. 'I'll keep it secret. And I'll buy a book. Two books! Three books! Because I saw one, Redso. Last year - I saw one in the market!' "

Congo Journey took O'Hanlon six years to write, mostly at night. He had now travelled through all the world's great jungle zones. Where to next? The answer, completely unexpected, was a trip above the Arctic circle, in the grip of a hurricane, in winter, on the Norlantean, a trawler so battered and rusted that he thought, when he first saw it, it was derelict.

In his earlier books, the literary skill was artfully concealed. He doesn't show off as a writer, and the structure seems so natural that only when you look very hard do you see how much has been left out so that each scene is properly framed. Trawler is different. All that it really has in common with his earlier travel books is that he spends a lot of time being sick in foetid places. The danger of the storm was real but it is also impersonal in a way that the snakes, the mosquitoes and the drunken soldiery of the earlier books were not.

He eats well and the hardships are on the other side of the hull. The marvellous creatures they examine on the gutting table are all safely dead. The monsters that really threaten the crew come out after the storm is over, when the exhaustion of days without sleep has eaten away the sanity and self-restraint of the men on board, and they talk about the things that really frighten them. And this crew of heroes, working at the extremity of their physical and mental strength, amid "that vast, unshaped, unrelenting violence that's out there, and out to get you, and that goes on for ever" - a boundless and voracious storm that might at any moment drown them, turns out to be frightened of only one thing - woman.

So the action of the book takes place less in the storm than in the long, shouted conversations afterwards, when physical exhaustion and sleep deprivation have reduced the whole crew to a sort of drunken lucidity and intimacy.

O'Hanlon talks about depression and horror, receives their confidences (all duly published) and dispenses enthusiastic advice. "You're a hero!" he tells his friend Luke Bullough, the biologist who got him on to the trawler. By this stage, everyone is shouting in italics and exclamation marks. These rough, stinking fishermen are pouring out their souls like Russian intellectuals.

"You must marry a district nurse!" O'Hanlon shouts. For Bullough, when he is not risking his life for science, or money, is a lifeboatman, risking his life for other sailors; and women can't stand it. His relationships are wrecked by the very thing that makes him heroically attractive, O'Hanlon decides: the only woman who can ever love him truly is one who has a beeper of her own, and who will be summoned by her own emergencies from his arms.

And this would be funny, shrewd and wise if it were all that happened. What makes the book, and the man, remarkable is that when you call Bullough today, he has now settled, with a district nurse.

Stockton, still a friend after all the horrors of their Amazonian journey, says: "Half the time I don't believe he exists, just as people who read that book don't believe that I exist. But I'm also a bit in awe of him. We all want to live in his shadow, because he's very illuminating."